Organic 5: Alcohols

Question 1

What is the general formula for alcohols?

Answer 1

ROH
CnH2n+1OH

Question 2

What is the bond angle of an OH group?

Answer 2

The bond angle of an OH group is 105 degrees
Because the 109.5 tetrahedral angle is reduced because of two lone pairs
Lone pairs repel more than bonding pairs

Question 3

Are alcohols soluble?

Answer 3

Shorter chain alcohols are soluble because the OH group can hydrogen bond to water molecules
Longer chain alcohols are insoluble because they have a larger non-polar hydrocarbon region, so cannot form hydrogen bonds

Question 4

What are the two methods of producing ethanol?

Answer 4

From crude oil
Fermentation

Question 5

Describe how ethanol is produced from crude oil.

Answer 5

Ethene is produced when crude oil fractions are cracked
Ethene can then by hydrated (in electrophilic addition reaction with water) to form ethanol

Question 6

Describe how ethanol is produced by fermentation, and give an equation.

Answer 6

Glucose is broken down using enzymes in yeast in anaerobic respiration
C6H12O6(aq) —> 2C2H5OH(aq) + 2CO2(g)

Question 7

How does temperature affect fermentation?

Answer 7

Enzymes can be denatured if temperatures are too high
But reaction will be too slow in low temperatures
So a compromise temperature of around 35 degrees is used

Question 8

Why must the fermentation vessels be kept anaerobic?

Answer 8

To prevent oxidation of ethanol to ethanoic acid

Question 9

How is ethanol distilled after fermentation?

Answer 9

Ethanol must be distilled by fractional distillation after fermentation
At boiling temperature 78 degrees Celsius
As aqueous solution produced by fermentation is impure

Question 10

Compare the two methods of producing ethanol.

Answer 10

Question 11

How can ethene be resynthesised from ethanol?

Answer 11

Dehydration

Question 12

What is the mechanism for dehydrating ethanol?

Answer 12

Acid catalysed elimination

Reagent: Excess conc H2SO4 or H3PO4
Conditions: Warm, under reflux

1) Lone pair of electrons from oxygen are attracted to H+ from acid
2) Electrons in C-H bond next to the carbon attached to the OH group move to the C-C bond, and form a double C=C bond
3) Electron pair in C-O bond move to the H2O+ group, which is removed to give ethene, H+ and water

Question 13

What are the advantages and disadvantages of making ethanol from crude oil?

Answer 13

Advantages
- Faster reaction
- Purer product
- Continuous process

Disadvantages
- High technology equipment needed
- Ethene is non-renewable resource
- High energy costs for producing high pressures

Question 14

What are the advantages and disadvantages of making ethanol from fermentation?

Answer 14

Advantages
- Sugar is a renewable resource
- Production uses low level technology

Disadvantages
- Batch process, so is slow
- Not continous
- Ethanol is not pure, so needs purifying by fractional distillation

Question 15

How is fermentation a carbon neutral process?

Answer 15

The carbon dioxide released is balanced by the carbon dioxide absorbed by the plant during photosynthesis

2 molecules are released in fermentation
4 molecules are released during combustion
6 molecules are absorbed during photosynthesis

Question 16

Why is ethanol production from fermentation renewable?

Answer 16

Because it comes from plants like sugar cane, which can be replanted

Question 17

What is ethanol usually used for?

Answer 17

Fuel e.g. in picnic stoves

Question 18

What can primary alcohols be oxidised into?

Answer 18

Primary alcohol + [O] –> aldehyde + H2O
Primary alcohol + 2[O] –> carboxylic acid + H2O

Question 19

Describe how you would carry out the oxidation of alcohols to aldehydes or ketones.

Answer 19

1) Using acidified potassium dichromate (K2Cr2O7 + H2SO4)
2) Orange dichromate (VI) ions are reduced to green chromium(III) ions
3) Heat and then distill off product to prevent further oxidation into carboxylic acid

Question 20

Describe how you would carry out the oxidation of primary alcohols to carboxylic acids.

Answer 20

1) Using acidified potassium dichromate (K2Cr2O7 + H2SO4)
2) Mixture is refluxed, so vapour condenses back into the flask, so all alcohol will be oxidised into acid

Question 21

Draw a diagram to show how you would distill off ethanal.

Answer 21

Question 22

Draw a diagram to show how you would carry out a reflux reaction for oxidising an alcohol to a carboxylic acid.

Answer 22

Question 23

Describe how you would carry out the Tollen’s test for aldehydes.

Answer 23

1) Add [Ag(NH3)2]+ (Tollen’s Reagent) to sample
2) Heat gently
3) Aldehydes will be oxidised into carboxylic acids
4) The silver ions in [Ag(NH3)2]+ will be reduced to silver atoms, forming a silver mirror
5) No visible change for ketones

Question 24

Give an equation to show how ethanal would react with Tollen’s reagent.

Answer 24

CH3CHO + 2Ag+ + H2O —> CH3COOH + 2Ag + 2H+

Question 25

Describe how you would carry out the Fehlings’ test for aldehydes.

Answer 25

1) Add Fehling’s solution containing Cu2+ ions
2) Heat gently
3) Cu2+ ions reduced to Cu+ in copper oxide, forming a red precipitate
4) No visible change for ketones

Question 26

Give an equation to show how Fehling’s solution would react with ethanal.

Answer 26

CH3CHO + 2Cu2+ + 2H2O —> CH3COOH + CU2O + 4H+

Question 27

How do you test for carboxylic acids?

Answer 27

1) Add sodium carbonate
2) Positive result = fizzing (carbon dioxide released)

E.g. Na2CO3 + 2CH3COOH —> 2CH3COONa + CO2 + H2O

Question 28

What is a biofuel?

Answer 28

A fuel derived from or produced from renewable biological sources e.g. plants

Question 29

What is a carbon neutral activity?

Answer 29

An activity that has no net carbon emissions (no emissions overall, i.e. same amount is removed as released)